I. Field of the Invention
This invention relates to the casting of molten metals, particularly molten metal alloys, by direct chill casting and the like. More particularly, the invention relates to such casting involving in-situ homogenization.
II. Background Art
Metal alloys, and particularly aluminum alloys, are often cast from molten form to produce ingots or billets that are subsequently subjected to rolling, hot working, and/or other treatments, to produce sheet or plate articles used for the manufacture of numerous products. Ingots are frequently produced by direct chill (DC) casting, but there are equivalent casting methods, such as electromagnetic casting (e.g. as typified by U.S. Pat. Nos. 3,985,179 and 4,004,631, both to Goodrich et al.), that are also employed. The term “direct chill” refers to the application of a coolant liquid directly onto a surface of an ingot or billet as it is being cast. The following discussion relates primarily to DC casting, but the same principles apply all such casting procedures that create the same or equivalent microstructural properties in the cast metal.
DC casting of metals (e.g. aluminum and aluminum alloys—referred to collectively in the following as aluminum) to produce ingots is typically carried out in a shallow, open-ended, axially vertical mold having a mold wall (casting surface) encircling a casting cavity. The mold is initially closed at its lower end by a downwardly movable platform (often referred to as a bottom block) which remains in place until a certain amount of molten metal has built up in the mold (the so-called startup material) and has begun to cool. The bottom block is then moved downwardly at a controlled rate so that an ingot gradually emerges from the lower end of the mold. The mold wall is normally surrounded by a cooling jacket through which a cooling fluid such as water is continuously circulated to provide external chilling of the mold wall and the molten metal in contact therewith within the casting cavity. The molten aluminum (or other metal) is continuously introduced into the upper end of the chilled mold to replace the metal exiting the lower end of the mold as the bottom block descends. With an effectively continuous movement of the bottom block and correspondingly continuous supply of molten aluminum to the mold, an ingot of desired length may be produced, limited only by the space available below the mold. Further details of DC casting may be obtained from U.S. Pat. No. 2,301,027 to Ennor (the disclosure of which is incorporated herein by reference), and other patents.
While usually carried out vertically as described above, DC casting can also be carried out horizontally, i.e. with the mold oriented non-vertically and often exactly horizontally, with some modification of equipment and, in such cases, the casting operation may be essentially continuous as desired lengths can be cut from the ingot as it emerges from the mold. In the caste of horizontal DC casting, the use of an externally cooled mold wall may be dispensed with. In the following discussion, reference is made to vertical direct chill casting, but the same general concepts apply to horizontal DC casting.
The ingot emerging from the lower (or output) end of the mold in DC casting is externally solid but is still molten in its central core. In other words, the pool of molten metal within the mold extends downwardly into the central portion of a downwardly-moving ingot for some distance below the mold as a sump of molten metal within an outer solid shell. This sump has a progressively-decreasing cross-section in the downward direction as the ingot cools and solidifies inwardly from the outer surface to form a solid outer shell until the core portion becomes completely solid. The portion of the cast metal product having a solid outer shell and a molten core is referred to herein as an embryonic ingot which becomes a cast ingot when it has fully solidified throughout.
As noted above, direct chill casting is normally carried out in a mold that has actively cooled walls that initiate the cooling of the molten metal when the molten metal comes into contact with the walls. The walls are often cooled by a primary coolant (normally water) flowing through a chamber surrounding the outer surfaces of the walls. When employed, such cooling is often referred to as “primary cooling” for the metal. In such cases, the direct application of first coolant liquid (such as water) to the emerging embryonic ingot is referred to as “secondary cooling”. This direct chilling of the ingot surface serves both to maintain the peripheral portion of the ingot in suitably solid state to form a confining shell, and to promote internal cooling and solidification of the ingot. The secondary cooling often provides the majority of the cooling to which the ingot is subjected.
Conventionally, a single cooling zone is provided below the mold. Typically, the cooling action in this zone is carried out by directing a substantially continuous flow of water uniformly around the periphery of the ingot immediately below the mold outlet, the water being discharged, for example, from the lower end of the cooling jacket provided for primary cooling. In this procedure, the water impinges with considerable force or momentum onto the ingot surface at a substantial angle thereto and flows downwardly over the ingot surface with continuing but diminishing cooling effect until the ingot surface temperature approximates that of the water.
U.S. Pat. No. 7,516,775 which issued on Apr. 14, 2009 to Wagstaff et al. discloses a process of molten metal casting of the above kind with an additional feature that the liquid coolant used for secondary (i.e. direct chill) cooling is removed from the exterior of the ingot at a certain distance below the mold outlet by means of a wiper, which may be an encircling solid elastomeric element through which the ingot passes or may alternatively be a wiper formed of jets of fluid (gas or liquid) directed countercurrent to the stream of secondary coolant liquid to lift the coolant streams from the ingot surface. The reason for removing the secondary coolant from the ingot surface is to allow the temperature of the outer solid shell of the embryonic ingot to rise and approach the temperature of the still-molten interior for a time sufficient to cause metallurgical changes to take place in the solid metal. These metallurgical changes are found to resemble or duplicating the changes that take place during conventional homogenization of solid castings carried out after casting and full cooling of such ingots. The rise in temperature of the shell following coolant wiping is due both to the superheat of the molten metal in the interior compare to the chilled metal of the solid outer shell, and to the latent heat that is generated as the molten metal of the interior continues to solidify over time. By this reheating effect, so-called “in-situ homogenization” is achieved, thereby avoiding the need for an additional conventional homogenization step following the casting operation. Full details of this procedure can be obtained from U.S. Pat. No. 7,516,775, the entire disclosure of which is specifically incorporated herein by this reference.
Although the in-situ homogenization procedure has proven to be most effective for its intended purpose, it has been found that certain metallurgical effects may materialize that, in some circumstances (e.g. when particularly large ingots are being cast), are undesirable. For example, as the solid shell of the ingot heats up following coolant wiping, it begins to expand at the internal interface between the solid and molten metal, thereby allowing metal of eutectic composition (the last molten metal to solidify) to pool in large pockets between previously-solidified grains or dendrites of metal of somewhat different composition present at the interface. The pooled metal of eutectic composition eventually solidifies to form large constituent particles of the metal that may be undesirably coarse for some applications. The removal of the secondary coolant by wiping also tends to change the characteristics of the molten metal sump (the central pool of molten metal in the embryonic ingot). This can lead to more severe changes in the chemistry across the ingot thickness, also called macrosegregation, than would be encountered in a standard DC ingot. If the partially solidified area between the fully liquid and fully solid regions, referred to as the semi-solid or mushy zone, becomes thicker, then solidification shrinkage induced flow will be enhanced. Solidification shrinkage induced flow occurs when the aluminum crystals (or crystals of other solvent metal) cool and begin to shrink. The shrinking crystals create a suction that pulls solute-rich liquid from high up in the mushy zone down into the small crevices at the bottom of the mushy zone. This phenomenon has the tendency to deplete the center of the ingot of solute elements while enriching the ingot or billet surface metal. Another phenomenon that affect is macrosegregation is called thermo-solutal convection; which is also enhanced by an increase in the thickness of the mushy zone. In thermo-solutal convection, liquid metal encountering the cold zone at the top of the sump near the mold wall and mold cooling sprays, becomes colder and denser. It sinks due to its increased density, and can travel through the upper part of the mushy zone, following the sump profile down and toward the center of the ingot. This phenomenon has the tendency to pull solute-rich liquid toward the ingot center, increasing the solute concentration at the ingot center and decreasing the solute at the ingot surface. A third phenomenon that affects macrosegregation is floating grains. The first crystals to solidify from an aluminum alloy are solute poor in systems with eutectic alloying elements. In the upper area of the mushy zone these crystals are loose and can be easily dislodged. If these crystals are pushed toward the bottom of the sump, as both gravity and thermo-solutal convection would be inclined to do, then the solute concentration in the ingot center will be reduced as these grains accumulate at the bottom of the sump. Again, this may be undesirable for certain applications.
U.S. Pat. No. 3,763,921 which issued to Behr et al. on Oct. 9, 1973 discloses direct chill casting of metals wherein coolant is removed from the ingot surface shortly below the mold, and reapplying the coolant to the ingot surface at a somewhat lower level. This is done to reduce ingot cracking and to permit high ingot casting speeds.
U.S. Pat. No. 5,431,214 which issued to Ohatake et al. on Jul. 11, 1995 discloses a cooling mold having first and second cooling water jackets provided inside the mold. A wiper is arranged downstream of the cooling mold to wipe off cooling water. A third cooling water jetting mouth is disposed downstream of the wiper. The disclosure focuses on smaller diameter billets.
It would be desirable to provide a modification of the in-situ homogenization process discussed above to minimize or overcome some or all of the unwanted effects when they are considered undesirable for applications for which the resulting cast ingots are intended.